Bulk solution synthetic methods often produce nanocrystals of multiple sizes and shapes, and hence there is relatively low yield of the desired size and shape. Murphy, C. J. Science 2002, 298, 2139-2141. Although colloid chemists have achieved excellent control over particle size for several metallic and semiconductor systems, there has been limited success in gaining control over the shape of the nanocrystals. Schmid, G.; Ed. Clusters and Colloids. From Theory to Applications; VCH: New York, 1994. Watzky, M. A.; Finke, R. G. J. Am. Chem. Soc. 1997, 119, 10382-10400. Jana, N. R.; Peng, X. J. Am. Chem. Soc. 2003, 125, 14280-14281. Controlling size, shape, and structural architecture of the nanocrystals requires manipulation of the kinetic and thermodynamic parameters of the systems via utilization of various additives, light and thermal energies, and their various combinations. Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924-1925. Pileni, M. P.; Ninham, B. W.; Gulik-Krzywicki, T.; Tanori, J.; Lisiecki, I.; Filankembo, A. Adv. Mater. 1999, 11, 1358-1362. Li, M.; Schnablegger, H.; Mann, S, Nature 1999, 402, 393-395. Jin, R.; Cao, Y. C.; Hao, E.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487-490. Sun, Y.; Xia, Y. Science 2002, 298, 2176-2179. Sun, Y.; Xia, Y. Adv. Mater. 2002, 14, 833-837. Sun, Y.; Mayers, B.; Herricks, T.; Xia, Y. Nano Lett. 2003, 3, 955-960.
Therefore, there remains a need for methods and compositions that overcome these deficiencies and that effectively provide shaped nanoparticles.
Surface enhanced Raman spectroscopy (SERS) is a powerful analytical tool for determining chemical information for molecules on metallic substrates. Moskovits, M. Rev. Mod. Phys. 1985, 57, 783-826. In general, there are two traditional operational mechanism to describe the overall SERS effect, electromagnetic (EM) and chemical (CHEM) enhancement mechanisms. EM enhancement is enhancement of the local electromagnetic field incident on an adsorbed molecule at a metallic surface. CHEM enhancement results from electronic resonance/charge transfer between a molecule and a metal surface, which leads to an increase the polarizability of the molecule. Otto, A.; Mrozek, I.; Pettenkofer, C. Surf. Sci. 1990, 238, 192. Schultz, S. G.; Janik-Czachor, M.; Van Duyne, R. P. Surf. Sci. 1984, 104, 419. Since the introduction of the SERS phenomenon on roughened silver electrodes, much attention has turned to SERS on spherical colloidal substrates of either gold or silver. Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. 1977, 84, 1-20. Albrecht, M. G.; Creighton, J. A. J. Am. Chem. Soc. 1977, 99, 5215-5217. Nie, S. M.; Emery, S. R. Science 1997, 275, 1102-1106. Krug, J. T.; Wang, G. D.; Emory, S. R.; Nie, S. M. J. Am. Chem. Soc., 1999, 121, 9208-9214. Freeman, R. G.; Bright, R. M.; Hommer, M. B.; Natan, M. J. J. Raman Spectrosc. 1999, 30, 733-738. Jensen, T. R.; Malinsky, M. D.; Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2000, 104, 10549-10556. Kneipp, K.; Kneipp, H.; Deinum, G.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Appl. Spectrosc. 1998, 52, 175-178. Colloidal nanoparticles are of interest as SERS substrates not only because they are strong light scatterers, but because of their tunable optical properties which depend on nanoparticle size, shape, and aggregation state. El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257-264. Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phzys. Chem. B 2003, 107, 668-677.
Spheroidal or rod-shaped nanoparticles are of significant interest as SERS substrates because of their tunable longitudinal plasmon bands and the “lightning rod” effect on surface enhancement. Schatz, G. C. Acc. Chem. Res. 1984, 17, 370-376. Gersten, J. I. J. Chem. Phys. 1980, 72, 5779-5780. While electric field enhancement is observed for 10-200 nM metallic particles, even greater local field enhancements are observed at sharp surface features, for example, at the tips of needle-shaped nanorods where the curvature radius is much smaller than the size of the nanoparticle. Gersten, J. I. J. Chem. Phys. 1980, 72, 5779-5780. This phenomenon is known as the lightning rod effect. Despite the desirable characteristics of metallic nanorods and nanowires as SERS substrates, only a few reports exist for SERS on rod- or wire-shaped nanoparticles. Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y, Yang, P. Nano Lett. 2003, 3, 1229-1323. Jeong, D. H.; Zhang, Y. X.; Moskovits, M. J. Phys. Chem. B 2004, 108, 12724-12728. Yao, J. L.; Pan, G. P.; Xue, K. H.; Wu, D. Y.; Ren, B.; Sun, D. M.; Tang, J.; Xu, X.; Tian, Z. Q. Pure Appl. Chem. 2000, 72, 221-228. Nikoobakht et al. have examined the use of unaggregated and aggregated gold nanorods as SERS substrates using pyridine and 4-aminothiophenol analytes. Nikoobakht, B. Wang, J. El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17-23. Nikoobakht, B., El-Sayed, M. A. J. Phys. Chem. A 2003, 107, 3372-3378. For SERS on unaggregated nanorods, the excitation wavelength was 1064 nm, far removed from the nanorod absorption bands (˜520 nm and 700 nm) where the EM enhancement mechanism is thought to be inoperative. Nikoobakht, B. Wang, J. El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17-23. Despite the off-resonance condition, appreciable SERS intensity was observed with surface enhancement factors (EF) of ˜104 for pyridine. The authors attributed the enhancement to a chemical (CHEM) enhancement mechanism of strongly adsorbed pyridine on the Au{110} surface of these nanorods. However, no reports have been made for SERS on nanorods where the Raman excitation occurs at a wavelength that overlaps with nanorod plasmon resonance, a condition where the EM enhancement mechanism should be operative.
Large enhancement factors and even single molecule SERS have been reported for molecules at junctions between aggregated nanoparticles. Jiang, J.; Bosnick, K.; Maillard, M.; Brus, L. J. Phys. Chem. B 2003, 107, 9964-9972. Xu, H. X.; Bjerneld, E. J.; Kall, M.; Borjesson, L. Phys. Rev. Lett. 1999, 83, 4357-4360. Michaels, A. M.; Jiang, J.; Brus, L. J. Phys. Chem. B 2000, 104, 11965-11971. This is a result of localized surface plasmon (LSP) coupling between nanoparticles and enhanced electromagnetic field intensity localized at nanoparticle junctions. Michaels, A. M.; Jiang, J.; Brus, L. J. Phys. Chem. B 2000, 104, 11965-11971. Vidal, F. J. G-.; Pendry, J. B. Phys. Rev. Lett. 1996, 77, 1163-1166. Wang, D.-S.; Kerker, M. Phys. Rev. B 1981, 24, 1777-1790. Markel, V. A.; Shalaev, V. M.; Zhang, P.; Huynh, W.; Tay, L.; Haslett, T. L.; Moskovits, M. Phys. Rev. B 1999, 59, 10903-10909. Su, K.-H.; Wei, Q.-H.; Zhang, X.; Mock, J. J.; Smith, D. R.; Schultz, S, Nano Lett. 2003, 3, 1087-1090. Atay, T.; Song, J-. H.; Murmikko, A. V. Nano Lett. 2004, 4, 1627-1731. Fromm, D. P.; Sundaramurthy, A.; Schuck, P. J.; Kino, G.; Moemer, W. E. Nano Lett. 2004, 4, 957-961. This LSP coupling between aggregated gold nanorods is believed to contribute to SERS enhancement observed by El-Sayed and coworkers. Nikoobakht, B., El-Sayed, M. A. J. Phys. Chem. A 2003, 107, 3372-3378. It is important to note, that although it is difficult to estimate enhancement factors for aggregated nanoparticles, the authors stated that SERS enhancements were always greater for aggregated gold nanorods than for aggregated spherical nanoparticles. Nikoobakht, B., El-Sayed, M. A. J. Phys. Chem. A 2003, 107, 3372-3378. Similarly, LSP coupling between colloidal nanoparticles and the surface of planar substrates, referred to as surface plasmon polariton (SPP), has also been well documented and has been reported for surface plasmon resonance (SPR) spectroscopy measurements. Shchegrov, A. V.; Novikov, I. V.; Maradudin, A. A. Phys. Rev. Lett. 1997, 78, 4269-4272. Holland, W. R.; Hall, D. G. Phys. Rev. B 1983, 27, 7765-7768. Kume, T.; Nakagawa, N.; Yamamoto, K. Solid State Commun. 1995, 93, 171-175. Lyon, L. A.; Musick, M. D.; Natan, M. J. Anal Chem. 1998, 70, 5177-5183. Lyon, L. A.; Pena, D. J.; Natan, M. J. J. Phys. Chem. B 1999, 103, 5826-5831. Hutter, E.; Cha, S.; Liu, J-F.; Park, J.; Yi, J.; Fendler, J. H.; Roy, D. J. Phys. Chem. B 2001, 105, 8-12. A 20-fold increase in signal is observed for biological sandwich assays where analytes are between nanoparticles and a planar surface, and LSP-SPP coupling occurs. LSP-SPP coupling has also been observed qualitatively by Zheng et al. between silver nanoparticles and surface plasmons of planar silver substrates. Zheng, J.; Zhou, Y.; Li, X.; Ji, Y.; Lu, T.; Gu, R Langmuir 2003, 19, 632-636. They observed greater SERS intensity for 4-aminothiophenol (4-ATP) self-assembled monolayers (SAMs) on silver when colloidal silver nanoparticles are adsorbed to the SAM than for the 4-ATP SAM of polished and electrochemically roughened silver. Zheng, J.; Zhou, Y.; Li, X.; Ji, Y.; Lu, T.; Gu, R Langmuir 2003, 19, 632-636.
A significant challenge in SERS on colloidal nanoparticle substrates is determining the number of analyte molecules sampled during the experiment. It is essential to calculate not only the number of nanoparticles in solution, but also the surface coverage of analyte molecules adsorbed to these nanoparticles. This is especially difficult for nanoparticles that are synthesized using strongly adsorbed capping agents including cetyltrimethylammonium bromide (CTAB), which may or may not be displaced by the analyte of interest. Nikoobakht, B. Wang, J. El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17-23. Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368-6374. In most reports, monolayer surface coverage on the nanocrystals is assumed, but if incorrect could lead to errors in calculations of EF values. For SERS on self-assembled monolayers (SAMs) on planar substrates, this problem is avoided altogether because there are no capping agents on these substrates and the number of molecules sampled is well known. Ulman, A. Chem. Rev. 1996, 96, 1533-1554. However, the tunability of the optical properties of planar SERS substrates is more difficult than solution-prepared colloids.
Previous research has demonstrated the high yield synthesis of gold nanorods and a plethora of other shapes of nanocrystals. Jana, N. R.; Gearheart, L. Murphy, C. J. Adv. Mater. 2001, 137, 1389-1393. Jana, N. R.; Gearheart, L. Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065-4067. Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414-6420. Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648-8649. Additionally, the immobilization of CTAB-protected gold nanorods on carboxylate-terminated SAMs has been studied. Gole, A.; Orendorff, C. J.; Murphy, C. J. Langmuir 2004, 20, 7117-7122. In the present invention, CTAB-capped nanoparticles of various shapes are immobilized on 4-mercaptobenzoic acid (4-MBA) monolayers. SERS spectra of 4-MBA are acquired to determine the effect of immobilizing gold nanoparticles on SERS of 4-MBA SAMS on gold and to determine whether the nanoparticle shapes, specifically their optical properties and surface structure, influence SERS of 4-MBA SAMs.
Therefore, there remains a need for methods and compositions that overcome these deficiencies and that effectively provide surface enhanced Raman spectroscopy.